Enhanced piezoelectric, pyroelectric and ferroelectric properties of poly(vinylidene fluoride) based nanocomposites containing dielectric nanofillers

Abstract

With the development of wearable electronic devices, the flexible functional materials have attracted much attention since they have excellent combination of mechanical properties and functionalities. Ferroelectric polymer is one of the most promising flexible materials with excellent piezoelectric, pyroelectric and ferroelectric properties and outstanding chemical resistance, thermal stability and processability, which are desirable characteristics for applications in sensors, transducers and actuators. In this thesis, ferroelectric polymer based nanocomposites with improved piezoelectric, pyroelectric and ferroelectric properties are investigated. As a typical semi-crystalline polymer, poly(vinylidene fluoride) (PVDF) exhibits piezoelectric and pyroelectric properties because of its polar crystalline ß phase. In this work, graphene oxides (GOs) with high surface activity are added to the PVDF to form different types of PVDF/GO nanocomposites. In the PVDF/GO thick films, it is found that GOs facilitate the crystallization of PVDF and the strong interaction between the -C=O groups in GOs and -CF₂ groups in PVDF could lead to the formation of ß phase. Moreover, a nearly pure crystalline ß phase is observed in the PVDF/GO nanocomposite subjected to a mechanical deformation which is only half of that for the pristine PVDF. The attachments of PVDF molecule chains to the GO nanosheets caused by the interaction between the -CF₂ or -CH₂ groups of PVDF and the functional groups of GOs are suggested to be responsible for the formation of piezoelectric ß-phase crystallites during the mechanical deformation. Remarkably, the dynamical polarization switching in GO and PVDF/GO nanocomposite films characterized by piezoelectric force microscopy (PFM) reveals that GO and PVDF/GO films might exhibit ferroelectric and piezoelectric properties with two-dimensional characteristics. In the graphene aerogel (GA) supported PVDF/GO nanoporous composites, the PVDF/GO is found to significantly improve the mechanical and electro-mechanical properties of GAs. The results on the improved piezoelectric and ferroelectric properties of PVDF/GO nanocomposites demonstrate that they are promising for electronic applications. The improved pyroelectric and electro-caloric properties of nanocomposites consisting of ferroelectric poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) and ferroelectric nano-fillers are investigated. In the nanocomposites consisting of P(VDF-TrFE) and Ba1-xSrxTiO₃ nanoparticles, an electro-caloric temperature change is as high as 2.5ºC under electric fields of ~600 kV cm⁻¹, which could result from the interfaces between the nanoparticles and the P(VDF-TrFE) matrix. In the nanocomposites consisting of P(VDF-TrFE) and BaTiO₃ nanowires, for the first time, a giant negative electro-caloric temperature change of -12ºC is observed. The results demonstrate that the nanocomposites possess great potentials in the electro-caloric solid state refrigeration because of their giant and tunable electro-caloric effects, excellent processability, high breakdown electrical strength and light weight.